Development of cost-effective techniques for recovering base and precious metals from their ores has been the goal of metallurgists for hundreds of years. Today, metallurgists are increasingly called upon to design processes for ores that are refractory to conventional recovery techniques. These challenges and the addition of environmental costs (including site remediation) to the total cost of mining have stimulated a search for alternatives to conventional methods for liberating precious metal values from sulfidic ores. An example of this need was highlighted at Randol Gold Forum '96 as follows (von Michaelis, H., "Gold-copper and copper-gold: Need for better processing technologies is urgent." Randol Gold Forum '96 Golden, CO: Randol International, 1996):
"There are more gold-copper and copper-gold ore deposits being discovered than ever before. Some of these are giant deposits, and they are located in all continents: Canada, USA, South America, Asia, Australia, Africa, and eastern Europe. The need for better processing technologies for treatment of copper-gold ores that do not respond to simple flotation is urgent and immediate."
There are three practical approaches to liberating gold from refractory ores in situations where the gold is intimately associated with sulfides: roasting, pressure oxidation (autoclaving) and bio-oxidation (Marsden, J. & House, I., The Chemistry of Gold Extraction. New York: Ellis Horwood, 1993). Roasting requires the construction and operation of an expensive and complex multiple-hearth or fluidized-bed furnace. Moreover, the process produces off-gases containing particulates and oxides of sulfur and arsenic that must be removed from the gas stream for both environmental reasons (e.g., prevention of acid rain) and for operational reasons. As an example, M. C. Robinson, D. W. Kirk and B. Jue (1988) in U.S. Pat. No. 4,789,529, Dec. 6, 1988, disclose a process for recovery of zinc from zinc-bearing sulfidic ores and concentrates by controlled oxidation roasting.
Pressure oxidation requires the construction of autoclave vessels that are operated at high temperatures (180 to 225.degree. C.) and pressures (1,500 to 3,200 kPa). These pressure vessels are considered to be "bombs" by many in the industry and concern about using highly pressurized vessels to process extremely corrosive slurries is widespread. For example, D. R. Weir and R. M. Genik-Sas-Berezowsky (1986) in U.S. Pat. No. 4,571,263, Feb. 18, 1986, discloses a process for recovery of gold from refractory auriferous iron-containing sulphidic concentrates that incorporates pressure oxidation. D. L. Jones in U.S. Pat. No. 5,223,024, Jun. 29, 1993, discloses a hydrometallurgical copper extraction process that incorporates agitated leaching at an elevated temperature and pressure. With both roasting and autoclaving, partial or selective oxidation of sulfides is not practical even in situations where it is not necessary to completely oxidize the sulfide to liberate the gold.
The remaining practical alternative is a bioprocess called bio-oxidation. Literally for centuries, the aerobic biological oxidation process (termed bio-oxidation) has been used by man to accelerate the solubilization of base-metal values in ores. The process has found particularly wide application in recovery of copper from ores and concentrates that contain copper-sulfide minerals and in recovery of uranium from its ores. For example, S. R. Zimmerley, D. G. Wilson and J. D. Prater in U.S. Pat. No. 2,829,924, Apr. 8, 1958, disclose a hydrometallurgical process for employing iron-oxidizing bacteria to regenerate a ferric sulfate, sulfuric acid lixiviant for leaching copper sulfide ores. The leach solution is aerated within a reservoir using "any suitable procedure for introducing oxygen and carbon dioxide into the solution" including "the bubbling of compressed air through the solution within the reservoir, the vigorous agitation of the body of the solution by mechanical means, and even, in some instances, the provision of extensive surface area for the reservoir relative to its depth." J. L. B. Aragones in U.S. Pat. No. 5,462,720, Oct. 31, 1995, discloses a process for leaching copper sulfides with a ferric-iron leach solution regenerated by "bacterial films of Thiobacillus ferrooxidans attached to an inert solid" in a bed of carrier material. E. T. Premuzic and M. S. Lin in U.S. Pat. No. 5,366,891, Nov. 22, 1994, disclose a method for biochemical solubilization of metal sulfides in geothermal sludge using Thiobacillus ferrooxidans and Thiobacillus thiooxidans mutants.
In bio-oxidation, aerobic, acidophilic, autotrophic bacteria, such as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Sulfolobus sp., are used to oxidize iron and sulfur minerals in which precious-metals are encapsulated or otherwise contained Ehrlich, H. L., & Brierley, C. L., Microbial Mineral Recovery. New York: McGraw-Hill., 1990). While bio-oxidation offers great promise due to its lower cost and reduced environmental impact, the ways in which it has been implemented in practice have generally made it impractical and too costly for large-scale application. Commercial process designs have been modeled on the century-old, abiotic, cyanidation process--a process with which hydrometallurgical engineers are very familiar. Bio-oxidation process designs, including biofilm reactors, slurry-pipeline reactors and fluidized-bed reactors, as well as process models are reviewed by Olsen, G. J. and Kelly, R. M. in "Microbiological metal transformations: Biotechnological applications and potential," (Biotechnology Progress (Vol. 2. No. 1), March, 1986).
A significant amount of work in the field of bio-oxidation and metals extraction has been accomplished by a variety of investigators. Tomizuka, N. & Yagisawa, M., in "Optimum conditions for leaching of uranium and oxidation of lead sulfide with Thiobacillus ferrooxidans and recovery of metals from bacterial leaching solution with sulfate-reducing bacteria," (Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena, Murr, L. E., Torma, A. E., & Brierley, J. A. (Eds.) New York: Academic Press, 1978), describe a two-step process for leaching of uranium and oxidation of lead sulfide where recovery of metals is accomplished by means of microbial sulfate reduction. Alper, J., in "Bacterial methods may strike it rich in refining metals, cleaning coal," (High Technology, April, 1984, pp. 32-35), describes the bio-oxidation of gold-bearing arsenopyrite/pyrite and notes that production of large amounts of arsenic and sulfurous gases is avoided. Torma, A. E., (Biotechnology: A Comprehensive Treatise in 8 Volumes, Deerfield Beach, Fla.: Verlag Chemie, 1988), reviewed bioleaching processes. Livesay-Goldblatt, E., (Fundamental and Applied Biohydrometallurgy, Proc. 6th International Symposium on Biohydrometallurgy, Vancouver, B.C. 89-96, 1986), described a process for gold recovery from arsenopyrite/pyrite ore by bacterial leaching and cyanidation. Torma, A. E., (Biotechnology: A comprehensive treatise in 8 volumes, Deerfield Beach, Fla.: Verlag Chemie, 1988), reviews bio-oxidation of gold and silver ores. Hackl, R. P., Wright, F., & Bruynesteyn, A., (Proceedings of the Third Annual General Meeting of Biominet, August 20-21, 71-90, 1986), described development of the BIOTANKLEACH process for leaching pyritic materials from gold and silver ore. The results of bench-scale and pilot-scale evaluations were presented. Marchant, P. B., & Lawrence, R. W., in "Flowsheet design, process control, and operating strategies in the bio-oxidation of refractory gold ores," (Proceedings of the Third Annual General Meeting of Biominet, August 20-21, 39-51, 1986), listed considerations in the design of commercial bio-oxidation plants. Lawrence R. W., in "Biotreatment of Gold," (Microbial Mineral Recovery New York: McGraw-Hill edited by Ehrlich, H. L. and Brierly, C. L, 1990), discussed biotreatment of gold ore. The benefits of using the BacTech moderately thermophilic cultures in bio-oxidation processes were discussed by Budden, J. R., & Spencer, P. A. in "Tolerance to temperature and water quality for bacterial oxidation: The benefits of BacTech's moderately thermophilic culture," (FEMS Microbiology Reviews, 11, 191-196, 1993). Chapman, J. T., Marchant, P. B., Lawrence, R. W., & Knopp, R., in "Biooxidation of a refractory gold bearing high arsenic sulphide concentrate: A pilot study," (FEMS Microbiology Reviews, 11, 243-252, 1993), described a modular mobile bioleach pilot plant for bio-oxidation of a refractory gold-bearing high-arsenic sulfide concentrate. Moffat, A. S., in "Microbial mining boosts the environment," (Science, 264, 778-779, 1994), disclosed how bio-oxidation can increase the efficiency of mining.
While most strains of T. ferrooxidans are considered to be mesophiles that grow optimally at about 35.degree. C., microbiologists have discovered facultative and obligate thermophilic iron- and sulfur-oxidizing bacteria, including Sulfolobus brierlevi, Sulfolobus acidocaldarius, Sulfolobus solfataricus, Sulfolobus BC and others. Thermophilic versus mesophilic bioleaching process performance was evaluated by Duarte, J. C., Estrada, P. C., Pereira, P. C., & Beaumont, H. P. (FEMS Microbiology Reviews, 11, 97-102, 1993). Two years of BIOX bio-oxidation pilot plant data were analyzed by Hansford, G. S., & Miller, D. M. in "Biooxidation of a gold-bearing pyritearsenopyrite concentrate," (FEMS Microbiology Reviews, 11, 175-182, 1993). Hoffman, W., Katsikaros, N., & Davis, G., in "Design of a reactor bioleach process for refractory gold treatment," (FEMS Microbiology Reviews, 11, 221-230, 1994), described the design of a reactor bioleach process for refractory gold treatment. Liu, X., Petersson, S., & Sandstrom, A., in "Evaluation of process variables in bench-scale bio-oxidation of the Olympias concentrate," (FEMS Microbiology Reviews, 11, 207-214, 1993), presented an evaluation of the effects of process variables on pyrite/arsenopyrite oxidation and gold extraction. Maturana, H., Lagos, U., Flores, V., Gaeta, M., Cornejo, L., & Wiertz, J. V., in "Integrated biological process for the treatment of a Chilean complex gold ore," (FEMS Microbiology Reviews, 11, 215-220, 1993), described an integrated biological process for treatment of a complex gold ore. Mineral sulfide oxidation by enrichment cultures of a novel thermoacidophilic bacteria were described by Norris, P. R. & Owen, J. P. in "Mineral sulphide oxidation by enrichment cultures of novel thermoacidophilic bacteria," (FEMS Microbiology Reviews, 11, 51-56, 1993). Rate controls on the bio-oxidation of heaps of pyritic material imposed by bacterial upper temperature limits were described by Pantelis, G. & Ritchie, A. I. M. in "Rate controls on the oxidation of heaps of pyritic material imposed by upper temperature limits on the bacterially catalyzed process," (FEMS Microbiology Reviews, 11, 183-190, 1993). Bio-oxidation bacteria have been characterized in detail. Brierly, C. L., & Brierly, J. A., in "A chemoautotrophic and thermophilic microorganism isolated from an acid hot spring," (Canadian J. Microbiology, 19, 183-188, 1973), characterized a chemoautotrophic and thermophilic (70.degree. C.) microorganism isolated from an acid hot spring. De Rosa, M., Gambacorta, A., & Bullock, J. D., in "Extremely thermophilic acidophilic bacteria convergent with Sulfolobus acidocaldarius," (J. General Microbiology, 86, 156-164, 1975), characterized the extremely thermophilic (85.degree. C.), acidophilic (pH 1.0) bacteria Sulfolobus acidocaldarius.
A number of investigators have characterized Thiobacillus ferrooxidans growth under anaerobic conditions. Pugh, L. H. and Umbreit, W. W. in "Anaerobic CO.sub.2 Fixation by Autotrophic Bacteria, Hydrogenomonas and Ferrobacillus," (Archives of Biochemistry and Biophysics, 115. 122-128, 1966), noted that "it is possible (for T. ferrooxidans) to achieve CO.sub.2 fixation under completely anaerobic conditions providing the oxidizable substrate (ferrous iron) is present." in recognizing the importance of removal of elemental sulfur that is produced during metal-sulfide oxidation, Brook, T. D. & Gustafson, J. in "Ferric Iron Reduction by Sulfur- and Iron-Oxidizing Bacteria," (Applied and Environmental Microbiology, 32. 567-571, 1976), suggested that "more rapid or effective leaching with ferric iron would be obtained if care were taken to develop and maintain a large active population of bacteria within a leach dump." Kelly, D. P. & Jones, C. A. in "Factors Affecting Metabolism and Ferrous Iron Oxidation in Suspension and Batch Cultures of Thiobacillus Ferrooxidans: Relevance to Ferric Iron Leach Solution Regeneration," (Basic Microbial Studies Applied to Leaching. 19-43, 1983), noted that "growing cultures (of T. ferrooxidans), whose growth ceases because of CO.sub.2 exhaustion, are still capable of oxidizing FeSO.sub.4 at a high rate for long periods." Brock, T. D., Smith, D. W., & Madigan, M. T. (Biology of Microorganisms. NJ: Prentice-Hall, Inc., 1984), noted "because of the huge dimensions of copper leach dumps, penetration of oxygen from air is poor, and the interior of these piles is usually anaerobic. Although most of the (oxidation) reactions . . . require molecular O.sub.2, it is also known that T. ferrooxidans can use Fe.sup.+3 as an electron acceptor in the absence of O.sub.2, and thus catalyze the oxidation reactions . . . anaerobically." Goodman, A. E., Babij, T. and Ritchie, A. I. M. in "Leaching of a sulfide ore by Thiobacillus ferrooridans under anaerobic conditions," (Recent Progress in Biohydrometallury, 361-376, 1983) Giovanni R. and Torma, A. E. (Eds.), Iglesias, Italy: Associazione Mineraria Sarda), compared aerobic and anaerobic batch leaching of a natural zinc-iron sulfide at pH 2.5. In their leaching experiments, they added nutrients and CO.sub.2 to the reactors, but did not add metal ions, such as Fe.sup.+2 ions or Fe.sup.+3 ions. Leaching of the zinc-iron sulfide under aerobic conditions resulted in production of acid, high numbers of bacteria being present in the supernatant, and a maximum of 48 percent of the iron in the ore being solubilized "and then it gradually precipitated out." Under aerobic conditions, "by the end of the run no iron was detected in solution." Leaching under anaerobic conditions produced "no precipitates or jarosite" and "no detectable acid," solubilization of 86 percent of the iron in the ore, and bacteria "firmly attached to the ore surfaces" with no bacteria in the supernatant. Under anaerobic conditions, CO.sub.2 concentrations were higher than can be achieved by contact with air.
During the last decade, processes for bio-oxidation of pyritic and arsenopyritic sulfides in gold and silver ores have been developed to the point of commercial application (see Torma, A. E., Biotechnology: A Comprehensive Treatise in 8 Volumes, Deerfield Beach, Fla.: Verlag Chemie, 1981). Recent improvements in the art are disclosed by: Hutchins et al. in U.S. Pat. No. 4,729,788, Mar. 8, 1988; Pooley et al. in U.S. Pat. No. 4,822,413, Apr. 18, 1989; Hacki et al. in U.S. Pat. No. 4,987,081, Jan. 22, 1991; Hunter in U.S. Pat. No. 5,076,927, Dec. 31, 1991; Brierly et al. In U.S. Pat. No. 5,127,942, Jul. 7, 1992; and Brierly and Hill in U.S. Pat. No. 5,246,486, Sep. 21, 1993.
When bio-oxidation is practiced in agitated reactors (by far the most common approach), large mass flow rates of oxygen and carbon dioxide are dissolved in slurries of finely-ground, flotation-concentrate particles. According to Marsden, J. & House, I., (The Chemistry of Gold Extraction. New York: Ellis Horwood, 1993), a commercial- scale, whole-ore treatment process has yet to be developed. Relatively inefficient oxygen and carbon dioxide dissolution methods are used, such as mechanical mixing and/or coarse-bubble aeration, because more efficient methods (e.g., fine bubble aeration or oxygenation in biofilters) are inappropriate (e.g., due to their tendency to clog with slurry particles, etc.). When injection of air or oxygen into the slurry is practiced, energy consumption is very high because the pressure at which the gas must be introduced (at the bottom of the reactors) is increased due to the high specific gravity of the slurry (p in lb/sq f=.gamma. in lb/cu ft * h in ft). When practiced in heaps, the mass transfer rate (via diffusion or convection) of oxygen into the heap limits the rate and extent of direct bio-oxidation.
While the above problems are serious, they are similar to those encountered in the aerobic cyanidation process itself, and efforts are underway to address them. Other problems raise "show-stopping" obstacles to adoption of the concept at a large scale. One major problem is a thermodynamic one. Bio-oxidation is an exothermic process. Oxidation of metal sulfides produces as much heat as do mechanical mixing of slurries and compression of gases. This heat must be removed from the reaction environment to prevent sterilization and/or boiling of the slurry. The magnitude of the waste heat (slurry cooling) problem (typically on the order of megawatts) has not escaped engineers charged with evaluating the feasibility of the approach (usually compared to roasting or autoclaving). The fact that the problem cannot be eliminate by repealing the first law of thermodynamics is also understood. Significantly, because it is difficult to remove the heat fast enough iron metal-sulfide concentrate slurries, pulp densities in the 10-25 percent solids range are used, and more tankage volume is required for bio-oxidation than is required for the cyanidation process which is operated at pulp densities in the 35-50 percent range. If the solids content of metal-sulfide slurries could be increased (e.g., in counter-current, upflow reactors), the capital (and maintenance) costs of the bio-oxidation process would be reduced, thus lowering the cost of gold recovery and making uneconomic reserves economic to mine.
A second major problem is that bio-oxidation as usually practiced typically results in the production of large mass flow rates of acidity (protons or H.sup.+ ions). This acidity must be neutralized in order to prevent sterilization of the slurry. Moreover, because the pH of the slurry must be elevated (to pH 10-11) prior to cyanidation, a large requirement for basicity (OH.sup.- ions) exists that must be met by addition of limestone, lime or sodium hydroxide. This, in turn, results in the production of large amounts of sludge that contains high concentrations of heavy metals and is difficult to dewater.
The above problems have existed for decades and persist today. They persist because system designers have not applied the principles of bioprocess engineering to solve them in an integrated, cross-disciplinary way. Moreover, process designers have not understood (and taken advantage of) all of the biocatalyzed reactions of the natural iron and sulfur cycles. Fortunately, there is a growing awareness within the industry that economic and regulatory (environmental) pressures will no longer allow nineteenth century approaches to these very real problems. The twenty-first century mineral processing challenges (very large operations, sulfidic ore bodies, environmental stewardship, etc.) will require new solutions--and biotechnologies will provide many of them.
With precious-metal ores, after metal-sulfide oxidation has occurred, precious metals are extracted from the ores. A great variety of precious-metal extraction processes have also been developed (see Gupta, C. K., & Mukherjee, T. K., Hydrometallurgy in Extraction Processes, Vol. I, Boston: CRC Press, 1990). Precious metal extraction processes are disclosed by: Pesic in U.S. Pat. No. 4,778,519, Oct. 18, 1988; Ball et al. in U.S. Pat. No. 4,902,345, Feb. 20, 1990; and Kandemir in UK Patent No. 2,180,829, published Apr. 8, 1987. F. J. Touro and T. K Wiewiorowski in U.S. Pat. No. 5,147,618, Sep. 15, 1992, disclose a process for recovering gold from refractory gold-bearing ores that uses sulfurous acid as the leaching agent. R. M. Hunter and F. M. Stewart in U.S. Pat. No. 5,449,397, Sep. 12, 1995, disclose an apparatus and method for biocatalyzed leaching of precious metals. The relatively low economic cost of cyanidation, however, has ensured its proliferation.
State-of-the-art precious metal heap leach practice varies with the nature of the ore. Biooxidation process steps may include ore crushing, acid pretreatment, inoculation with appropriate sulfide-oxidizing bacteria, addition of nutrients, recirculating the biolixiviant and cooling the heap (for 3 to 8 days), and allowing the heap to "rest" (for 3 to 8 days). Precious metal extraction by means of cyanidation may include the process steps of washing the heap for an extended period (e.g., 14 days) to remove residual acidity or iron content, breaking the heap apart in order to agglomerate it with cement and/or lime to make a new heap, leaching it with an alkaline cyanide or thiosulfate solution for 30 to 40 days, and recovery of gold and silver from the leach solution by absorption on activated carbon or zinc dust precipitation.
A variety of less-widely practiced methods of metal-sulfide oxidation are available in the prior art patent literature. M. Dubrovsky in U.S. Pat. No. 5,238,662, Aug. 24, 1993, discloses processes for recovering precious metals that incorporate molten salt chlorination. M. Dubrovsky and P. J. Marcantonio in U.S. Pat. No. 5,104,445, Apr. 14, 1992, disclose a process for recovering metals from refractory ores that involves chlorination of an ore concentrate in the presence of solid salt at a temperature between 300 and 650.degree. C. K. J. Fair, G. van Weert and J. C. Schneider in U.S. Pat. No. 5,013,359, May 7, 1991, disclose a process for recovering gold from refractory sulfidic ore that involves using nitric acid as an oxidizing agent.
No single prior art reference or combination of references have suggested combining available knowledge to practice biocatalyzed anaerobic metal-sulfide oxidation as proposed herein. The prior art does not teach the use of anaerobic processes to solubilize base metals from metal sulfides using aerobically-regenerated, oxidized metal ions and to liberate (mobilize) precious-metals, such as gold, silver and platinum-group elements from their ores and concentrates. In fact, the prior art teaches away from the present invention toward aerobic processes for leaching of metals from ores and concentrates. Such aerobic processes are disclosed in the following recently published books on the subject: Ehrlich, H. L. (1990), Microbial Mineral Recovery, New York: McGraw-Hill; Gupta, C. K., & Mukhedee, T. K. (1990), Hydrometallurgy in Extraction Processes, Vols. I and II, Boston: CRC Press; Yannopoulos, J. C. (1991), The Extractive Metallurgy of Gold, New York: Van Nostrand Reinhold; Marsden, J. & House, I. (1993), The Chemistry of Gold Extraction, New York: Ellis Horwood. The disclosures in the aforementioned patents are incorporated by reference herein as if fully set forth.